Server 2012 R2 Essentials and Exchange Online – How tied are they?

A bit of background first:

An SBS 2008 server went down with Hard Drive failure. Re-built and all now working. Customer was, however, concerned about interruption of emails if it happened again so migrated all emails to Exchange Online.

They now want to replace the Server and Server 2012 R2 Essentials has been decided as the best option.

I am aware that you can “tie-in” Exchange Online with User Accounts on Server 2012 Essentials and create & delete etc.

My question is:

Does the Server running 2012 Essentials have to be functional to log into Exchange online if the account is "linked"?

I remember, when migrating from SBS 2008 that I read you could do something similar but came to the conclusion that authentication to the email account took place on the server and, if the server wasn't running, you couldn't log in and get your email by whatever means – online, Outlook etc. Since the company moved to Exchange Online to avoid such an issue, I never went any further with it.

I was therefore wondering if this is the same for 2012 Essentials or whether the two can be looked after independently of each other when it comes to email and Office 365 as a whole.

A case of convenience rather than control...

Thanks in advance for your advice.
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Cliff GaliherCommented:
The Office 365 Integration in Essentials is still cloud based. Accounts and passwords are synced, but authentication for O365 is based on Azure AD, not the server.
Markieboy1OwnerAuthor Commented:
Hi Cliff and thanks again for your quick reply.

Just to clarify, and as an example, I can create a new user in Essentials. Link it with Exchange Online and the account is also created in Exchange Online ( or Office 365 ) for email.

If the server died, the user could still get email and control of the Office 365 account could still be done through the Office 365 control panel - change password, deletion etc.?
Cliff GaliherCommented:
Correct. If you plan on bringing the server back, I'd avoid making changes to avoid confusing the sync, but access would be fine,

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Markieboy1OwnerAuthor Commented:
Excellent! I can now go ahead and sort as needed. Thanks again for your quick response.
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